1. Field of the Invention
The invention is based on a resistor which is designed in the form of a column, according to the preamble of patent claim 1 . The invention also relates to a method for producing such a resistor.
A resistor of this type is used for measurement, protection or control tasks in medium- or high-voltage systems. In general, this resistor is designed as a non-linear resistor (varistor) and has a cylindrical resistor body which is arranged between two electrodes, aligned parallel, and is made of a ceramic material. The ceramic material in general comprises a zinc oxide doped specifically with chosen elements, such as Bi, Sb, Co and Mn, and is produced by dense sintering of a pressed body at temperatures between 1000 and 1300xc2x0 C.
The varistor is preferably used in overvoltage suppressors and must be specified such that it can carry, without damage, current pulses of 65 or 100 kA produced by lightning strikes or switching operations. Such current pulses are applied in the course of the manufacturing process to the electrodes of the varistor in order to check their resistance to high current. The amplitude, the form and the duration of typical current pulses and apparatuses for carrying out tests with such current pulses are described, for example, in IEC Standard 99-4, Part 4: Metal-oxide surge arresters without gaps for a.c., first edition 1991-11, Bureau Central de la Commission Electrotechnique Internationale [Central Bureau of the International Electrotechnical Commission], Geneva, Switzerland.
2. Discussion of Background
A resistor of the type mentioned initially is specified in EP 0 196 370 A1. This resistor has a cylindrical, ceramic resistor body on a doped zinc-oxide base. The mutually parallel, planar end surfaces of the resistor body are metallized and are connected in DC terms to two connecting fittings, one of which is connected to high-voltage potential and the other to earth potential. The resistor is part of an overvoltage suppressor having only one resistor. Since this resistor is fitted with the connecting fittings, there is no need for a suppressor housing. The resistor has a length which is considerably greater than its diameter and can thus be loaded directly with voltages of more than 10 kV. However, if high-current pulses produced by a lightning strike or switching operations then occur, then it is not possible to preclude failure of the resistor and thus of the overvoltage suppressor as well.
Accordingly, one object of the invention as it is specified in patent claims 1 and 4 is to provide a novel resistor of the type mentioned initially which is distinguished by great length and in which, after loading with high-energy current pulses, failure can reliably be precluded, and to specify a method using which such a resistor can be manufactured in a simple and cost-effective manner.
The resistor according to the invention has a great length with a relatively small diameter and can be loaded with highly energetic current pulses without the strength of its ceramic material being exceeded. This prefabrication of the resistor, which is advantageous for cost-effective manufacture of a device containing the resistor, preferably an overvoltage suppressor, is based on the effect that a thermal impulse caused by a highly energetic current pulse leads to sudden heating of the ceramic material. The ceramic material when heated in a pulsed manner expands thermally to a large extent. To do this, it requires a time period governed by the speed of sound in it. If this time period is in the order of magnitude of the duration of the current pulse, then severe stresses are formed in the ceramic which, in a long resistor, form tensile forces which act predominantly in the axial direction and exceed the strength of the ceramic material beyond a specific resistor length. Thus, for a given ceramic material strength and a given pulse load, the length of the resistor must not exceed a specific value. Since the thermal effects of the current pulse are in general reduced as the volume of the resistor increases, the resistor may be made longer with increasing diameter for the same pulse load.
A preferred method for producing a resistor according to the invention is distinguished by the following method steps:
A characteristic graph is determined for resistors made of the same ceramic material and with the same diameters, but with different lengths.
Mechanical stresses produced in the ceramic material by loading it with at least one highly energetic current pulse are shown on the characteristic graph as a function of the length of the resistors.
A given electrical field strength and at least one current pulse of defined amplitude, form and duration are assigned as electrical parameters to each characteristic.
Sample resistors designed and dimensioned in a corresponding manner to the resistors on the characteristic graph are loaded with the electrical parameters assigned to a characteristic.
Finally, after being loaded with the electrical parameters, the sample resistors are analyzed for their re-usability.
When this method is carried out in practice, two sample resistors of different length must be assigned to one of the characteristics, one of which is intact and a second is defective after being loaded with the electrical parameters, and the strength of the ceramic material must furthermore be entered as a normalization variable between the two sample resistors, and an area of the characteristic graph underneath the normalization magnitude must then be chosen in order to define a mechanical stress capacity which is still permissible, and thus to define a length which is still permissible for the resistor which can be loaded with the electrical parameters.